1. Field of the Invention
The present invention relates to an optical network for carrying TDMA (Time Division Multiple Access) signals and to transmitters and receivers for use in nodes of such a network.
A network embodying the present invention might be used, for example, as a local area network (LAN) for interconnecting computer systems. The increasing power of computer systems in terms of processor speeds and storage capacity has made it possible for conventional personal computers to handle multimedia applications involving real time video and animation and computer graphics. The high bandwidth data associated with such applications place heavy demands on the network and the performance of conventional LANs has failed to keep pace.
2. Related Art
An optical network using synchronous TDMA potentially offers a far higher bandwidth, and so might be used as a high speed LAN to replace a conventional LAN. However, in existing optical networks, while signal transmission has been carried out in the optical domain, in practice some electronic circuits have been required for such functions as channel selection. It has been recognised that such electronic components of the network infrastructure constitute a bottleneck restricting the performance of the network.
xe2x80x9cA High Speed Broadcast and Select TDMA network Using All-Optical Demultiplexingxe2x80x9d, L. P. Barry et al, ECOC ""95 pp 437-440, describes an experimental OTDM network. At the receivers in the network nodes, an optical clock signal is detected and a variable delay applied in the electrical domain to the detected clock signal to select a particular TDMA channel. After pulse shaping, the signal is taken back into the optical domain by driving a local optical source, a DFB laser, which produces an optical signal for use in a subsequent all-optical switching stage.
The paper by Prucnal et al, xe2x80x9cUltrafast all-optical synchronous multiplex access fibre networksxe2x80x9d, IEEE Journal on Selected Areas in Communications, SAC-4, no. 9, December 1986 proposes an alternative approach in which different delays, and hence different TDMA channels, are selected in the optical domain. The optical signal is split between different paths each having a different characteristic delay and an electro-optic gate in each path is controlled so that the signal passes only through the path having the desired delay.
According to a first aspect of the present invention, there is provided an optical network comprising:
a) an optical transmission medium; and
b) a plurality of nodes connected to the optical transmission medium, each of the plurality of node including a respective dark pulse generator which is coupled in-line with the optical transmission medium and in series with the others of the dark pulse generators and which is arranged to generate dark pulses in an optical signal carried on the transmission medium.
In this specification, a xe2x80x9cdark pulsexe2x80x9d is a temporal gap, or region of reduced intensity radiation, in an essentially continuous burst of optical radiation, or light beam. An advantage of using dark pulses in place of bright pulses is that optical signal generation is simplified, as will be discussed in the subsequent description. Also, while pulse alignment remains important for dark pulse OTDM, to minimise cross-talk, the extinction ratio necessary for successful dark pulse OTDM transmission is typically smaller than that required for bright pulse OTDM.
Preferably each node further comprises a variable delay stage which is arranged to apply a variable delay to a network clock signal in the electrical domain and which is connected at its output to the dark pulse generator.
The inventors have found it to be particularly advantageous to use in combination dark pulse generation and channel selection in the electrical domain. This further simplifies node structures, whilst enabling effective operation at high bit rates, for example at 40 Gbit/s.
Preferably each node further comprises a clock receiver for receiving a network clock signal carried on the optical transmission medium, the clock receiver including a photoelectric detector for converting the clock signal to the electrical domain.
Preferably the electro-optic modulator is an electro-absorption modulator (EAM).
The present inventors have found that significant advantages can be achieved by combining channel selection in the electrical domain with the use of an electro-optic switch with a fast non-linearity to read the selected channel. In particular, relatively high switching rates can be achieved without the power losses typically associated with all-optical channel selection. It is found to be particularly advantageous to use an EAM. The fast response time of such a device makes possible a switching window as short as a few picoseconds. The receiver as a whole is therefore capable of operating at bit rates of 40 Gbit/s or higher.
Preferably the receiver includes means for separating the clock signal in the optical domain from the received TDMA datastream. Preferably the said means for separating comprise a polarising beam splitter, in use the clock signal being marked by a different polarisation state to the TDMA datastream.
Preferably a first output of the means for separating is connected to the optical input of the electro-optic modulator, in use TDMA data passing from the first output to the modulator, and a second output of the means for separating is connected to the detector, in use optical clock signals passing from the second output to the detector.
Preferably an impulse generator is connected between the output of the variable delay stage and the control input of the electro-optic modulator.
The electro-optic modulator may require a drive signal having somewhat shorter pulses than those output by the delay stage. In this case advantageously some form of pulse shaping may be used, and in particular the output of the delay stage may be applied to an electrical impulse generator. This may be a device using step recovery diodes to generate short electrical pulses from a sine wave.
Preferably the variable delay stage comprises a plurality of logic gates, means connecting a first input of each gate to an input path for the clock signal, control means connected to a second input of each gate, and means connecting outputs of the gates in common to an output path for the delayed clock signal, the said means connecting inputs and outputs of the gates to respective input and output paths being arranged to provide paths of different respective lengths via different gates, in use the control means applying control signals to the gates to select a path and a corresponding delay for the clock signal.
This preferred feature of the present invention uses an array of logic gates to provide an electronic channel selector suitable for an integrated construction, and capable of quick reconfiguration. This channel selector is not limited in applicability to receivers in accordance with the first aspect of the present invention, but may be used with other receiver designs, or in node transmitters. In particular, it may be combined with a local optical source in a receiver in which an all-optical switch was used in place of the electro-optic modulator of the first aspect of the invention.
Preferably at least one of the said means connecting inputs and outputs comprises a microstrip delay line. Preferably the means connecting inputs and outputs comprise a pair of microstrip delay lines and the gates are connected between the pair of microstrip delay lines.
Preferably adjacent connections to the gates on the microstrip delay line on the input side of the gates are separated by a path length corresponding to t/2 and adjacent connections on the microstrip delay line on the output side of the gates are separated by a path length corresponding to t/2, in use the gates being controlled to vary the delay by multiples of t, where t corresponds to the channel spacing in the time domain of the TDMA signal.
Preferably the optical transmission medium is an optical bus, and more preferably hs an optical bus topology.
As set out in further detail in the description of the embodiments below, the use of dark pulse generation is found to be particularly well-adapted to a network using a bus-topology. This allows the dark pulse generators in the different nodes to be effectively coupled in series so as to build up an OTDM multiplex. At the same time, the bus topology eliminates many of the timing problems associated with other topologies, such as star networks.
According to a second aspect of the present invention there is provided a method of operating an optical network including a plurality of nodes connected to an optical transmission medium, the method comprising:
a) at one of the plurality of nodes, imposing dark pulses representing a data stream on an optical signal which is carried on the optical transmission medium; and
b) at a subsequent node, receiving the optical signal including the dark pulses imposed in step (a) and imposing dark pulses on the optical signal in a different respective time slot, thereby creating an OTDM (optical time division multiplexed) signal.